An afterloading apparatus is used for inserting an energy emitting source, for example a radioactive source from a reference position through a catheter tube into a desired location within an animal body for the purpose of radioactive treatment of for example cancer. The radioactive source at the end of the source wire must be handled with extreme care. Even short exposures at close distances can result in radiation injury. It is therefore extremely important that the afterloading apparatus, which controls the advancement and retraction of the source wire within the catheter tube, operates with a high reliability, and that it is configured to position the source wire within the catheter tube and the animal body in a controllable manner and with extreme accuracy and precision.
In the past, the control means of the afterloading apparatus which position the distal end of the source wire within the catheter tube have included special optical or mechanical sensors for sensing, when that distal end is located at a home or reference position within the afterloading apparatus.
Although such optical and mechanical sensors have operated generally satisfactorily in detecting the presence of the source wire's distal end at its reference position within the afterloading apparatus, the sensors' performance can degrade over time. This degradation is due, in part, to debris accumulating at the site of the sensor and to radiation damage in case of an opto sensor. One source of such debris is the catheter tube itself. As the source wire is cycled into and out of the catheter tube, a certain amount of catheter material is scraped away, and this material is drawn into the afterloader's drive mechanism. This debris can obscure the view of an optical sensor. An another disadvantage of the prior art systems is that small cable diameters of the drive wire can not obscure completely the light emitted by an opto sensor and therefore the tip of this cables can not be referenced exactly.